Nonvolatile semiconductor storage device such as a NAND flash memory device has found application in a wide range of products such as digital cameras, mobile terminals, portable audio, and SSD (Solid State Drives) which is an emerging storage solution to replace hard disk drives conventionally used in large volume storage applications for mobile computers.
Flash memory device is typically known to employ a stacked gate structure of floating gate type in which a control gate electrode is stacked above a floating gate electrode.
With advances in microfabrication and densification, semiconductor devices are being fabricated based on design rules that go beyond the resolution limits of today's lithography process. Driven by such tight design rules, the so-called cross coupling effect is increasing its influence on device properties which can be typically observed through variation in threshold voltage when programming a memory cell located adjacent to another memory cell.
The variation in threshold voltage, that is, expanded distribution of program threshold voltage is typically induced by: too much or too less impurity dope, dimension and shape variance, and program noise within the memory cell elements. The expanded distribution of program threshold leads to greater negative impact on the speed and reliability of operation.